Radiotherapy is the main treatment modality for nasopharyngeal carcinoma. However, ionizing radiation is itself a known carcinogen, and sarcomas can be a complication of treatment. Radiation-induced osteosarcomas in the head and neck are particular rare. Imaging studies of radiation-induced sarcomas in patients with head and neck carcinoma have been limited to case reports and relatively small case series. We report an unusual case of a radiation-induced osteosarcoma of the mandibular ramus following radiotherapy for nasopharyngeal carcinoma with computed tomography (CT) and magnetic resonance imaging (MRI). These findings can be helpful for differentiating osteosarcoma from other tumors of the mandible.